Certain PUFAs are important biological components of healthy cells and are recognized as: “essential” fatty acids that cannot be synthesized de novo in mammals and instead must be obtained either in the diet or derived by further elongation and desaturation of linoleic acid (LA; 18:2 ω-6) or α-linolenic acid (ALA; 18:3 ω-3); constituents of plasma membranes of cells, where they may be found in such forms as phospholipids or triacylglycerols; necessary for proper development (particularly in the developing infant brain) and for tissue formation and repair; and precursors to several biologically active eicosanoids of importance in mammals (e.g., prostacyclins, eicosanoids, leukotrienes, prostaglandins). Additionally, a high intake of long-chain ω-3 PUFAs produces cardiovascular protective effects (Dyerberg et al., Amer. J. Clin. Nutr. 28:958-966 (1975); Dyerberg et al., Lancet 2(8081):117-119 (1978); Shimokawa, H., World Rev. Nutr. Diet 88:100-108 (2001); von Schacky et al., World Rev. Nutr. Diet 88:90-99 (2001)). Numerous other studies document wide-ranging health benefits conferred by administration of omega-3 and/or omega-6 PUFAs against a variety of symptoms and diseases (e.g., asthma, psoriasis, eczema, diabetes, cancer).
Today, a variety of different hosts including plants, algae, fungi and yeast are being investigated as means for commercial PUFA production. Genetic engineering has demonstrated that the natural abilities of some hosts (even those natively limited to LA and ALA fatty acid production) can be substantially altered to result in high-level production of various long-chain ω-3/ω-6 PUFAs. Accordingly, production of arachidonic acid (ARA; 20:4 ω-6), eicosapentaenoic acid (EPA; 20:5 ω-3) and docosahexaenoic acid (DHA; 22:6 ω-3) all may require expression of either the Δ9 elongase/Δ8 desaturase pathway (which operates in some organisms, such as euglenoid species, and which is characterized by the production of eicosadienoic acid (EDA; 20:2 ω-6) and/or eicosatrienoic acid (ETrA; 20:3 ω-3)) or the Δ6 desaturase/Δ6 elongase pathway (which is predominantly found in algae, mosses, fungi, nematodes and humans and which is characterized by the production of γ-linoleic acid (GLA; 18:3 ω-6) and/or stearidonic acid (STA; 18:4 ω-3)) (FIG. 1).
The Δ8 desaturase enzymes identified thus far have the ability to convert both EDA to dihomo-γ-linolenic acid (DGLA; 20:3 ω-6) and ETrA to eicosatetraenoic acid (ETA; 20:4 ω-3) (wherein ARA are EPA are subsequently synthesized from DGLA and ETA, respectively, following reaction with a Δ5 desaturase, while DHA synthesis requires subsequent expression of an additional C20/22 elongase and a Δ4 desaturase; FIG. 1).
Based on the role Δ8 desaturase enzymes play in the synthesis of, e.g., ARA, EPA and DHA, there has been effort to identify and characterize these enzymes. Initial efforts on the isolation and characterization of Δ8 desaturases from Euglena gracilis and several sequence variations within the Euglena gracilis Δ8 desaturase have been reported (see, e.g., Wallis et al., Arch. Biochem. and Biophys. 365(2):307-316 (1999); PCT Publication No. WO 2000/34439; U.S. Pat. No. 6,825,017; PCT Publication No. WO 2004/057001). Also, Applicants' Assignee's co-pending U.S. Patent Application No. 2006/0195939 and U.S. Pat. No. 7,256,033 disclose amino acid and nucleic acid sequences for a Euglena gracilis Δ8 desaturase. In other work, commonly owned, co-pending U.S. patent application Ser. Nos. 11/635,258 and 11/951,697 describe a synthetically engineered mutant Δ8 desaturase, derived from Euglena gracilis. PCT Publication No. WO 2005/103253 discloses amino acid and nucleic acid sequences for a Δ8 desaturase enzyme from Pavlova salina (see also U.S. Publication No. 2005/0273885). Sayanova et al. (FEBS Lett. 580:1946-1952 (2006)) describes the isolation and characterization of a cDNA from the free living soil amoeba Acanthamoeba castellanii that, when expressed in Arabidopsis, encodes a C20 Δ8 desaturase. Also, Applicants' Assignee's co-pending U.S. patent application Ser. No. 11/737,772 discloses amino acid and nucleic acid sequences for a Δ8 desaturase enzyme from Pavlova lutheri (CCMP459) whereas U.S. Patent Application No. 2008/0095915 discloses amino acid and nucleic acid sequences for a Δ8 desaturase enzyme from Tetruetreptia pompquetensis CCMP1491, Eutreptiella sp. CCMP389 and Eutreptiella cf—gymnastica CCMP1594. Applicants' Assignee's co-pending U.S. patent application Ser. No. 12/099,799 discloses amino acid and nucleic acid sequences for a Δ8 desaturase enzyme from Euglena anabaena. 
Based on the utility of expressing Δ8 desaturases in conjunction with Δ9 elongases, there has also been effort to identify and characterize Δ9 elongases from various sources. Most Δ9 elongase enzymes identified so far have the ability to convert both LA to EDA and ALA to ETrA (wherein DGLA and ETA are subsequently synthesized from EDA and ETrA, respectively, following reaction with a Δ8 desaturase; ARA and EPA are subsequently synthesized from DGLA and ETA, respectively, following reaction with a Δ5 desaturase; and DHA synthesis requires subsequent expression of an additional C20/22 elongase and a Δ4 desaturase; FIG. 1). A Δ9 elongase from Isochrysis galbana has been publicly available (described in GenBank Accession No. AAL37626, as well as PCT Publication No. WO 02/077213). Applicants' Assignee's co-pending U.S. Patent Application No. 2007/0118929 discloses a Δ9 elongase from Eulgena gracilis. Applicants' Assignee's co-pending U.S. Patent Application No. 2007/0117190 discloses a Δ9 elongase from Eutreptiella sp. CCMP389. Applicants' Assignee's co-pending U.S. patent application Ser. No. 12/102,979 (filed Apr. 15, 2008) discloses amino acid and nucleic acid sequences for a Δ9 elongase enzyme from Euglena anabaena. 
Most delta-5 desaturase enzymes identified so far have the primary ability to convert DGLA to ARA, with secondary activity in converting ETA to EPA (where DHA is subsequently synthesized from EPA following reaction with an additional C20/22 elongase and a delta-4 desaturase). The delta-5 desaturase has a role in both the delta-6 desaturase/delta-6 elongase pathway and the delta-9 elongase/delta-8 desaturase pathway (FIG. 1). Furthermore, based on the role delta-5 desaturase enzymes play in the synthesis of e.g., ARA, EPA and DHA, there has also been an effort to identify and characterize these enzymes from various sources. As such, delta-5 desaturases have been disclosed in both the open literature (e.g., GenBank Accession Nos. AF199596, AF226273, AF320509, AB072976, AF489588, AJ510244, AF419297, AF07879, AF067654 and AB022097) and the patent literature (e.g., U.S. Pat. Nos. 5,972,664 and 6,075,183). Applicants' Assignee's co-pending U.S. Patent Application No. 2007/0271632 discloses a 45 desaturase from Peridinium sp. CCMP626 whereas Applicants' Assignee's co-pending U.S. Patent Application No. 2007/0292924 discloses a Δ5 desaturase from Euglena gracilis. Applicants' Assignee's co-pending U.S. patent application Ser. No. 12/111,228 (filed Apr. 23, 2008) discloses amino acid and nucleic acid sequences for a Δ5 desaturase enzyme from Euglena anabaena. 
Applicants' Assignee has a number of patent applications concerning the production of PUFAs in oleaginous yeasts (e.g., Yarrowia lipolytica), including: PCT Publication Nos. WO 2004/101757 and WO 2004/101753; U.S. Patent Application Nos. 2006/0115881, 2006/0094092, and 2006/0110806; and U.S. patent application Ser. No. 12/061,738 (filed Apr. 3, 2008).
Relatedly, PCT Publication No. WO 2004/071467 (published Aug. 26, 2004) concerns the production of PUFAs in plants.
Despite the disclosures cited above, there is a need for additional genes encoding polypeptides having Δ9 elongase activity as it is mainly through genetic variation that a wide variety of host cells may be optimized for PUFA production.